1. Field of the Invention
Apparatuses consistent with the exemplary embodiments relate to an encoder for measuring displacement of a moving object, and more particularly, to an optical encoder including a light-emitting unit and a light-receiving unit.
2. Description of the Related Art
An encoder is an electro-mechanical device that converts displacement of a moving part into an analog or digital signal. The encoder is mainly installed at a rotating shaft and is used to measure a rotation angle or speed of the rotating shaft.
Encoders are largely classified into incremental encoders and absolute encoders.
An incremental encoder includes a light-emitting unit, a disc including a track having slits arranged at a regular interval, and a light-receiving unit for detecting light transmitted through the slits. The incremental encoder measures a rotation angle or speed of the disc by counting pulse signals obtained by the light-receiving unit.
However, if a power supply of the incremental encoder is cut off, the rotation angle of the disc cannot be immediately checked even when power is reintroduced to the incremental encoder. In this case, in order to check the rotation angle of the disc, the disc should be rotated until a reference slit formed in the disc is detected by the light-receiving unit.
An absolute encoder includes a light-emitting unit, a disc including a plurality of tracks each having slits arranged to form a circle, and a light-receiving unit for detecting light transmitted through the slits. The slits that constitute one track have a same width, and the slits that constitute a different track have a different width. A plurality of light-receiving units are provided so that each of the plurality of light-receiving units can respectively corresponds to each of the tracks and can detect whether light emitted by the light-emitting unit passes through the slits of each track. The absolute encoder calculates the rotation angle of the disc from information obtained by the plurality of light-receiving units.
Even if a power supply of the absolute encoder is cut off, the rotation angle of the disc can be immediately checked once power is reintroduced to the absolute encoder. However, since the disc of the absolute encoder should include a plurality of tracks disposed in the form of concentric circles, there is a difficulty in reducing the size of the disc. Thus, there is a difficulty in making a small-sized absolute encoder. Also, since a plurality of light-receiving units should be disposed in each of the tracks, manufacturing costs of absolute encoders increase.